The R9 280X is quite a bit more powerful than the GTX 660 so the R9 280X would bottleneck your CPU the least. The GTX 770 is pretty much the equivalent of the R9 280X's power. The 770 is also quite a bit better than the 660 and is slightly better than a GTX 680 so I wouldn't even look into getting a 660 if you have the option to buy the R9 280X for the same price.